1. Field of the Invention
The invention relates to a clad steel pipe which is furnished with a clad material comprising a corrosion resistant material on an internal surface or an external surface of a steel pipe as a base material, and more particularly, to a structure for reducing the amount of using the clad material.
2. Description of the Related Art
In general, a steel pipe has a property excellent in strength but a disadvantage of being poor in corrosion resistance. Therefore, the steel pipe is provided with a measure of corrosion resistance in view of the using environments and others, and then used. In particular, when a high corrosion resistance is demanded, a solid pipe made of a corrosion resistant material such as a stainless steel is provided instead of a steel pipe.
However, the solid pipe made of the corrosion resistant material is expensive in itself and inferior in strength compared with a steel pipe. For securing the strength of the solid pipe, the amount of employing materials should be increased, but the result is very expensive.
Accordingly, as a high corrosion resistant steel pipe for solving those problems, a clad steel pipe has been developed and broadly used, which is furnished with a clad material including the corrosion resistant material on an internal surface or an external surface of the steel pipe which is the base material.
Referring to FIGS. 3 and 4, conventionally, JP-A-58-154488 sets forth a method of curving the clad steel plate or the clad steel band 21 in a width direction to shape a pipe, for example, as shown in FIG. 3, and welding both edges 22 to be a seam part in a width direction of the clad steel plate or the clad steel band 21, thereby to manufacture a clad steel pipe 20.
In the clad steel plate or band 21, as shown in FIG. 4, the height of a base plate of both edges 22 to be the seam part in the width direction is formed to be thinner than other parts of both edges 22, and at the same time, a clad material 23 to be covered by both ends 22 is formed to be thicker than the clad material 23 other than both edges 22 in the width direction, and as a whole, a uniform thickness is provided.
In the conventional clad steel pipe 20 as shown in FIGS. 3 and 4, the thickness A (FIG. 4) of the clad material 23 of both edges 22 to be the seam part in the width direction of the clad steel plate or band 21 is larger than the thickness B (FIG. 4) of the clad material 23 at other than both edges 22. Thereby, such an arrangement absorbs a discrepancy caused by welding both edges 22 in the width direction of the clad steel plate or band when manufacturing the clad steel pipe 20 (hereafter called the "discrepancy at the welded part when manufacturing the clad steel pipe 20").
It is accordingly possible to prevent the corrosion resistance from deterioration caused by the discrepancy at both edges of the clad material 23 in the seam part of the clad steel plate or band 21.
However, on the other hand, no consideration is paid at all to the absorption of a discrepancy caused when welding the clad steel pipes to each other in a construction field of laying clad steel pipes. That is, such a discrepancy as shown with dotted lines in FIG. 5 caused by dimensional tolerance or out of roundness (hereafter called the "discrepancy at the welded part when laying the clad steel pipes 20").
For absorbing the discrepancy at the welded part when executing work on the clad steel pipes, the thickness of the clad material at both ends in the length direction of the clad steel pipe must be determined to be large values having a margin for the amount of an expected discrepancy.
Considering this fact, by application to the conventional clad steel pipe 20, since the thickness of the clad material 23 is, as shown in FIGS. 6A and 6B, uniform in the length direction of the clad steel pipe 20, the thickness B of the clad material 23 other than at both edges 22 in the width direction, to be a seam part of the clad steel plate or band 21, must be determined to be a value which can absorb the discrepancy in the welded part when executing the laying of the clad steel pipe 20.
In short, in the conventional clad steel pipe 20, attention is given only to the absorption of the discrepancy at the welded part when manufacturing the clad steel pipe 20, while no attention is paid to the absorption of the discrepancy at the welded part when laying the clad steel pipe 20. If the clad material 23 has a large thickness (defined by a thickness A) in both edges 22 at the seam part of the clad steel plate or band 21, it is possible to suppress the amount of use of the expensive clad material 23.
But in view of the actual execution where many clad steel pipes 20 are connected to one another by welding in the laying construction field in the length direction over several kilometers to several tens of kilometers in total extension, the discrepancy at the welded part when laying the clad steel pipe 20 must by all means, be absorbed.
In addition, in the welding works in the laying field where the equipment and the working circumstances are largely restrained, the discrepancy at the welded part when laying the clad steel pipe 20 should be also absorbed in order to select a suitable welding method to effect a high working efficiency.
Therefore, in the structure of the conventional clad steel pipe 20, the thickness B of the clad material 23 other than at both edges in the width direction to be the seam part of the clad steel plate or band 21, is determined to be the value which can absorb the discrepancy at the welded part when laying the clad steel pipe 20, and the thickness A of the clad material 23 of both edges in the width of the clad steel plate or band 21 is determined to be larger than the thickness B.
Such a situation does not reduce the amount of use of the expensive clad materials 23 but increases it in proportion to the difference between the thickness A and the thickness B. Besides, since the thickness A and the thickness B of the clad materials 23 are uniform in the length direction of the clad steel pipe 20 as shown in FIG. 6B, the amount of use of the clad materials 23 may be probably heightened several tens of times or several hundreds of times depending on the laying distance, inviting a tremendous increase in cost.
With respect to the clad steel pipe 20 and corrosion resistance, a margin for corrosion is not necessary when the clad material is correctly selected and sizes of the clad material 23 provide no influence with respect to the corrosion resistance.
Therefore, in spite of only both edges 22 for the seam part in the width direction of the clad steel plate or band 21 being used in the conventional clad steel pipe 20, the forming of the clad material 23 to be thick denotes a very ineffective use of many clad materials 23 which make no contribution to the corrosion resistance, and gives impetus to cost increases.